Breast cancer is a leading cause of death among women. Malignant breast tumors that are not cured by initial surgery typically respond poorly to traditional chemotherapy and further antiestrogen therapy. New therapeutic strategies that will inhibit growth and suppress progression of breast cancer cells need to be identified. This goal might be achieved through targeting of signaling pathways used by the most prominent peptide hormones that regulate breast cancer growth and differentiation. Based on novel preliminary data that extend the work supported by this grant, we provide compelling rationale for investigating the growth-regulatory role of the prolactin-activated Jak2 tyrosine kinase/Stat5 transcription factor pathway in normal and malignant human breast epithelial cells. Importantly, we have discovered loss of Stat5 activation during progression from normal to primary cancer and to metastatic disease. We have also discovered a novel prolatin-activated Jak1-Stat3 signaling pathway that is Jak2-independent, and appears to require the interleukin-6 signal transducer, gp130. We will use specialized molecular tools and technologies to test the following two central hypotheses: a) Prolactin inhibits growth of normal and malignant human breast epithelial cells through activation of the Jak2-Stat5 signaling pathway. b) Prolactin activates an alternative, breast cancer-specific Jak1-Stat3 pathway by a mechanism that requires the interleukin-6 signal transducer, gpl30. We are well qualified to successfully undertake the proposcd studies. We expect to establish the importance of the Jak2-Stat5 pathway in growth-suppression of untransformed and malignant breast epithelial cells, and to identify the mechanism by of aberrant prolactin activation of the Jak 1-Stat3 pathway in breast cancer cells. The research is significant and important because it will provide novel insight into the biological role and mechanisms of prolactin signal transduction in breast cancer. More effective therapeutic strategies for breast cancer could be a result.